The present invention relates generally to a system and method for controlling imbalances between load and supply in power systems, and more particularly to a load shedding system and method for controlling imbalances in a power system.
In power systems of today, it is essential that a balance between the amount of power being used (“load”) and the amount of power being supplied (“supply”) be maintained to provide a secure and reliable power system, thereby preventing blackouts and power interruptions. Every imbalance between the supply and load causes a deviation of the frequency. More importantly, if the load exceeds the supply and the deficit is not counteracted, the deficit in supply can lead to system interruptions and even blackouts.
One way to control these imbalances is to use load shedding. Load shedding (i.e., opening a circuit breaker to reduce the load) has been used to mitigate imbalances between supply and load since the infancy of power systems, and has basically remained unchanged over the last 100 years. During the same period, the logic used to shed the load has also only changed slightly. Unfortunately, the power systems, expectations of the users, and their loads have changed drastically, thereby necessitating better ways to handle these imbalances.
Currently, there are basically two kinds of under frequency load shedding schemes: the first is based on pre-determined frequencies encountered during a frequency decline, and the second is based on the rate of decline of the frequency (i.e., first derivative of frequency with respect to time). Both schemes use different time delays (from zero delay to several seconds, in some cases).
The first scheme drops the load based on predefined frequencies, f1, f2, f3, . . . fn in the hope that the dropped load would be enough to reverse the frequency decline by narrowing the gap between the supply and the connected load. The predefined set points are based on previous studies, and they do not consider the actual state of the system when the disturbance occurs or any dynamic system conditions ensuing from the disturbance. Though this scheme is relatively good when there is a smaller disturbance coupled with a long time to respond, it is unable to differentiate between some “normal” system oscillations and actual system disturbances. Hence, it may not shed enough load to arrest a frequency decline associated with real disturbances. This could cause the disturbance to cascade and result in the system actually collapsing because the appropriate action (shed enough load) was not taken at the right time.
The second scheme for shedding load is based on the rate of change (first derivative) of the frequency decline (combined often with predefined frequencies). This scheme solves some of the concerns with the first scheme above by providing a means for a faster response to disturbances and a better method for differentiating disturbances from “normal” system oscillations. The rate of change of the frequency (df/dt) is used at some or all of the predefined frequencies.
Unfortunately, the last blackouts, including the 2003 North East Blackout, have shown that the above existing load shedding schemes are not sufficient to arrest frequency declines and can not save power systems from large disturbances. Accordingly, there is a need for a load shedding scheme that can sufficiently arrest frequency declines that result from large disturbances.